1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, printer, or facsimile apparatus which forms an image by an electrophotographic method.
2. Description of the Related Art
These days, image forming apparatuses using the electrophotographic method are achieving higher speeds and higher qualities. In particular, color image forming apparatuses require accurate color reproduction and tint stability, and generally have a function of automatically controlling the image density.
In image density calibration control, an image density detector incorporated in an image forming apparatus detects a plurality of test toner images (patches) which are formed on an image carrier while changing image forming conditions. The detected toner images are converted into a substantial amount of toner adhesion, and optimum image forming conditions are decided based on the conversion result.
A plurality of types of image density calibration control operations is generally executed to obtain optimum values for a plurality of types of image forming conditions. The types of image forming conditions include conditions such as the charging voltage, exposure intensity, and developing voltage, and a lookup table setting used to convert a signal input from the host into output image data when forming a halftone image. The tint varies depending on a change of the environment where an image forming apparatus is used, the use log of various consumables, and the like. The image density calibration control needs to be periodically executed to always stabilize the tint.
According to the detection principle of an optical image density detector, a light receiving element receives light which is emitted from a light emitting element and reflected by a patch or image carrier itself. The amount of toner adhered to the patch is calculated from the received light. Conversion into a substantial amount of toner adhesion is executed based on the relationship between an output from the light receiving element when no toner is adhered to the image carrier, and an output from the light receiving element when toner is adhered to the image carrier.
The reflectance of the image carrier surface changes depending on the position of the image carrier. To calculate the amount of toner adhesion with high precision, outputs in the presence and absence of toner need to be acquired at the same position on the image carrier. In general, a background output VB from the light receiving element in the absence of toner is acquired at a specific position. Then, the image carrier rotates at least one round. A patch is formed at the same position to acquire a patch output VP from the light receiving element. The background output VB corresponds to light reflected by the background of the image carrier. The patch output VP corresponds to light reflected by the patch. Specifying the position on the image carrier requires the circumference of the image carrier. This is because the time taken for a specific position on the image carrier to rotate is obtained by dividing the circumference by the circumferential speed (process speed) of the image carrier.
However, the circumference of the image carrier changes depending on variations of components, the environment of the image forming apparatus, and the like. If the circumference is used as a fixed value, an error occurs in specifying a position. To prevent this, information associated with the circumference of the image carrier needs to be measured dynamically.
There is proposed the following method for an image forming apparatus which employs an intermediate transfer method. More specifically, a mark is attached to the surface of an intermediate transfer member. An optical sensor receives light reflected by the mark to measure the circumference of an image carrier. The mark is attached not to an image-formed surface used for image formation, but to a longitudinal end on the intermediate transfer member.
Japanese Patent Laid-Open No. 2002-214854 proposes the following technique based on the fact that the intermediate transfer belt rotates one round every time the driving roller for driving the intermediate transfer belt rotates 5.2 times. More specifically, the eccentric component of the facing roller cycle is obtained. A cycle profile reflecting thickness nonuniformity of the intermediate transfer belt is attained from the eccentric component. The facing roller is arranged to face an optical sensor via a driven belt. In Japanese Patent Laid-Open No. 2002-214854, accurate density detection is done based on the attained cycle profile. In this manner, there has conventionally been known a technique of obtaining, in consideration of the influence of a driving roller, the dynamic characteristic (e.g., density characteristic) of an apparatus that may vary owing to aging factors and environmental factors.
However, the conventional technique considers the eccentric component of a driving roller, but suffers the following problems. For example, when the image forming apparatus operates for a long time, mold dust and transfer roller dust are generated upon wear. Such a foreign substance may enter the gap between the facing roller and the image carrier, and be adhered to the facing roller. If the image carrier is irradiated with light to detect the reflected light in this state, the influence of the foreign substance appears in the detection result every time the facing roller rotates once.
The foreign substance adhered to the facing roller adversely affects the detection result of light coming from a detection target. This inhibits obtaining an accurate light detection result or an accurate dynamic characteristic of the apparatus that is calculated from the light detection result.